The present invention relates to an optical system for a focus detecting device that is suitable to an optical apparatus such as a single-lens reflex camera.
FIG. 1 shows a conventional single-lens reflex camera 10. The camera 10 is provided with a taking lens 51 as a main lens to form an image onto a film 12. A TTL (Through-The-Lens) passive focus detecting device 21 is installed in a bottom portion of a camera body 11 of the single-lens reflex camera 10. A part of light passed through the taking lens 51 passes through a half-mirror portion 14 of a main mirror 13, and then the light is reflected by a sub-mirror 15 toward the focus detecting device 21. In FIG. 1, x-, y- and z-directions are defined. The z-direction is parallel to an optical axis of the taking lens 51, the x-direction is parallel to an equivalent optical axis of the taking lens 51 after reflection by the sub-mirror 15, and the y-direction is perpendicular to both of the x- and z-direction.
The focus detecting device 21 includes a box 21a in which a condenser lens 31, a mirror 25, a pair of separator lenses 27a and 27b and a line sensor 29 are arranged. The line sensor 29 has a great number of pixels that are aligned with the y-direction. The optical path in the focus detecting device 21 is also shown in FIG. 2 in a developed fashion. An opening 23 to define a focus detecting area is bored on the box 21a. The opening 23 is located at an equivalent film plane 17. The light entered from the opening 23 is converged by the condenser lens 31 and then reflected by the mirror 25. The separator lenses 27a and 27b divide the light reflected from the mirror 25 into two portions and form images on different areas 29a and 29b on the line sensor 29. In FIG. 2, light 21LC passes through the central area of the opening 23 and light 21LO passes through the peripheral area of the opening 23. Entrance pupils of the focus detecting device 21 is represented by ovals 21P. The entrance pupils 21P are defined as areas that are optically conjugate with the entrance pupils of the separator lenses 27a and 27b through the condenser lens 31 and the opening 23.
Since the correlation between the positions of the images formed on the line sensor 29 represents focusing condition of the taking lens 51 with respect to the film 12, the focusing condition can be detected by calculating output signals from the line sensor 29.
Downsizing of the camera requires a compact design of the focus detecting device 21, which also requires decreasing the size of the box 21a. On the other hand, a predetermined optical path length must be kept in order to prevent overlapping of the separated images on the line sensor 29. If the optical path extends along a diagonal line of the box, it makes the most effective use of the confined space in the box of the focus detecting device.
FIG. 3 shows an optical system in an improved focus detecting device 22. A condenser lens 31 is provided at an opening 23 bored to a box 22a. The condenser lens 31 is decentered to the opposite side of a line sensor 29 with respect to the center of the opening 23, which deflects light 21L incident in the focus detecting device 22 toward the opposite direction to the line sensor 29. The deflected light is reflected by the mirror 25 to be incident on the line sensor 29 through the separator lenses 27a and 27b. FIG. 4 shows the deflecting function of the decentered condenser lens 31. The rectangular opening 23 is located at the peripheral portion of the condenser lens 31 as shown in FIG. 5 when the opening 23 is viewed along the optical axis O of the condenser lens 31.
The mirror 25 is diagonally opposite to the line sensor 29, which makes the optical path between the mirror 25 and the line sensor 29 extend along the diagonal line of the box 22a. This therefore makes the most effective use of the confined space in the box 22a of the focus detecting device 22.
However, the focus detecting device 22 has a disadvantage of mismatch between an exit pupil of the taking lens 51 and entrance pupils 21P of the focus detecting device 22 due to coma. The decentered arrangement of the condenser lens 31 causes coma in the light, and particularly, the longer the distance from the optical axis O is, the larger the coma becomes. Since the coma changes a deflecting angle of the passing light, the deflecting angle of the light 21LO passing the periphery of the opening 23 is larger than the deflecting angle of the light 21LC passing the center of the opening 23 as shown in FIG. 6. It is therefore, the entrance pupils 21P of the focus detecting device 22 are curved such that the peripheral portions bend upward as shown in FIG. 7, which may cause the mismatch between the exit pupil 51P of the taking lens 51 and the entrance pupils 21P of the focus detecting device 22.
In the example of FIG. 7, outside portions of the entrance pupils 21P represented by hatching are not within the exit pupil 51P, which results in the difference between the brightness distribution detected by the line sensor 29 and the actual brightness distribution, causing error in the focus detection. Thus, the decentered arrangement of the condenser lens 31 in the focus detecting device 22 only allows a narrow focus detection area in the center of the equivalent film plane 17 in order prevent the error in the focus detection.